If your plants look healthy but simply are not developing bulbs it could be that the soil is not loose and permeable as it can be. Try adding organic matter such as compost or aged-manure to loosen it up a little more. If your soil is already amended with organic matter, it might be that you are planting your onions at the wrong time of year (plant onion sets in the spring and allow them to grow all summer - when the tops turn yellow the underground bulbs are beginning to mature).